From the point of view of air pollution control and environmental concerns, there is seen a need to decrease the quantities and concentrations of carbon dioxide (CO2) emissions resulting from the combustion of coal, oil, and other carbon fuels. For this purpose, methods for removing carbon dioxide from the flue gas (also commonly referred to as “exhaust gas”) resulting from such combustion are presently being developed. One such method employs absorption of CO2 from flue gases using aqueous solutions of solvents. Examples of solvents include amine-containing solutions. Examples of amines include, but are not limited to, for example, alkanolamine, monoethanolamine and the like, and combinations and/or mixtures thereof, which are hereinafter referred to as “amines” or “amine compounds.”
An example of a solvent-based method for removing CO2 with the use of amines is provided in U.S. Pat. No. 5,318,758, which is incorporated by reference herein in its entirety. The '758 patent proposes a method which performs decarbonation by using an aqueous solution of an amine compound as a solution for absorbing carbon dioxide from the flue gas within an absorber column.
In general, a solvent-based method for removing CO2 from a flue gas includes a flue gas supplied by a combustion gas supply blower, which is cooled by a cooling tower, and then fed to an absorption column. In the CO2 absorption section of the absorption column, the fed flue gas is brought into countercurrent contact with an absorbing solution supplied through an absorbing solution supply port via at least one nozzle. As a result, CO2 in the flue gas is absorbed and removed by the absorbing solution. The loaded absorbing solution, which has absorbed CO2, is sent to a regeneration tower by the absorbing solution discharge pump through an absorbing solution discharge port. In the regeneration tower, the loaded absorbing solution is regenerated, and fed again to the absorption tower through the absorbing solution supply port.
Most solvent-based CO2 capture processes involve an exothermic reaction between the solvent and the flue gas, which leads to a temperature profile in the absorption column. Depending upon the process parameters, the maximum temperature (also known as “temperature bulge”) in the column could be in the top, bottom or middle section of the absorber column Owing to this temperature increase, there are some solvent losses that occur in the process along the column. These solvent losses occur mainly through the decarbonated flue gas that is leaving the top of the absorber column.
In a CO2 capture process with a chemical solvent, such as amines, a wash section may be included on top of the absorber column to reduce this emission loss. The decarbonated flue gas contacts the wash water in the wash section on top of the absorber, which captures some of the solvent from the gas phase and is recovered in the liquid phase. This recovered solvent in the liquid phase can be either used directly in the CO2 absorption process or sent to a solvent make-up section.
Depending upon the gas separation processes upstream of the CO2 absorber column, the flue gas entering the absorber column is mostly saturated. In order to maintain the solvent capacity, it is important to run the process without any solvent losses as well as water accumulation. Any accumulation of excess water in the absorber column will lead to the dilution of the solvent concentration, which will impact its mass transfer characteristics and variation of partial pressures along the column. Hence it is important to ensure that the amount of water entering and leaving the process is very close, which is called “water neutrality”. Previously known and described wash sections have not provided reduced solvent neutrality. The processes and systems described herein are seen to address at least these issues.